The present invention relates to an exposure control technique which uses both an electronic shutter function of controlling the timing of start of charge accumulation or reading of charge in an image-pickup device and a mechanical shutter function of covering a light-receiving surface of the image-pickup device with a light-shielding blade.
An image-taking apparatus such as a digital camera has a CCD of a progressive scan type (hereinafter referred to as PS-CCD) as an image-pickup device. With the PS-CCD used as the image-pickup device, the image-taking apparatus can control the exposure state of image data provided from output of the image-pickup device by controlling charge accumulation time from elimination of charge to transfer of accumulated charge. When the PS-CCD is used as the image-pickup device, the image-taking apparatus can adjust the charge accumulation time without using any mechanical shutter, but it is preferable to include a mechanical shutter for reducing the occurrence of smear. However, the occurrence of smear cannot be prevented since luminous flux reaches the PS-CCD even in a short time period from the completion of the charge accumulation operation in the PS-CCD to shielding of light by the mechanical shutter. Various approaches have been proposed to reduce the occurrence of smear.
On the other hand, a CMOS image sensor, which is an image-pickup device of an XY address type, has the advantage of negligibly reduced smear as compared with the CCD. Since the CMOS sensor has been technically developed for a larger size, it is often used in a digital camera of a single-lens reflex type requiring a large image-pickup device which can easily provide high-quality image data.
The CMOS image sensor of the XY address type, however, accumulates charge in different timings for different rows as a so-called rolling shutter, and thus cannot finish the accumulation operation simultaneously in all pixels. If exposure control is attempted by controlling the charge accumulation time in the CMOS image sensor, the accumulation period in the first row of scan lines is different from that in the final row approximately one frame, so that it is contemplated that the CMOS image sensor is not suitable to take a still image of a moving subject. Thus, a mechanical shutter is used for controlling the exposure time in the CMOS image sensor.
Reset operation in each row for starting charge accumulation in the CMOS image sensor is performed the time for charge accumulation before the timing of reading operation of the signal level of accumulation charge in each row. The speed of the reset operation can be different from the scan speed of the read operation of the signal level of accumulation charge. As an example of using this feature, Japanese Patent Laid-Open No. 11-41523 has disclosed an apparatus which performs exposure control by performing reset operation in a CMOS image sensor one row at a time at a speed in accordance with the travel of a mechanic shutter. The apparatus disclosed in Japanese Patent Laid-Open No. 11-41523 performs the reset operation one row at a time at the speed in accordance with the travel of the mechanical shutter to start charge accumulation, and shields light with the mechanical shutter, and then performs the read operation of the signal level of accumulated charge one row at a time. The exposure control of image data can be achieved by adjusting the interval between the reset operation and the travel of the mechanical shutter. Since the reset operation is performed one row at a time at the speed accordance with the travel of the mechanical shutter, the difference in the accumulation time between the first row and the final row of scan lines can be improved to the same level as that when a mechanical shutter is used which is provided with a light-shielding blade serving as a front curtain (hereinafter referred to as a front blade) and a light-shielding blade serving as a rear curtain (hereinafter referred to as a rear blade). According to the structure, it is possible to reduce smear in taking a moving image due to the use of the CMOS image sensor and to provide a high-speed shutter in which the CMOS takes a still image of a moving subject.
The front blade and the rear blade of the mechanical shutter, however, are typically driven by a spring and are often held at the position of start of travel through absorption with an electromagnet. For this reason, the mechanical shutter does not always move along the same curve which represents its travel characteristic (hereinafter referred to as a travel curve) because of a plurality of factors such as variations in the position of the image-taking apparatus, temperature, humidity, driving voltage of the electromagnet for holding the mechanical shutter, variations in mechanical shutters, and changes over time.
To perform the reset operation at a proper timing in associated with the travel of the mechanical shutter, it is necessary to provide a detection system which detects the travel curve of the mechanical shutter and a feedback system which controls the timing of the reset operation in accordance with the detection result. An apparatus disclosed in Japanese Patent Laid-Open No. 2005-159418 has a plurality of photointerrupters arranged in the travel direction of a front blade to detect the travel curve of the front blade from output of the photointerrupters when the front blade travels. FIG. 13 shows the structure. Reference numeral 93 shows the front blade, 94 a rear blade, and 96 an image-pickup device. Reference numeral 90 shows a blade detection mechanism which includes a plurality of photointerrupters 90-1 arranged in the travel direction of the front blade 93. The photointerrupter 90-1 includes a light emitter 91-1 and a light receiver 92-1 disposed on both sides of the front blade. The photointerrupter 90-1 detects the movement of the front blade to determine the travel curve of the front blade. The travel curve is relied on to estimate the travel curve of the rear blade, reset operation is performed, and the rear blade shields light. The structure can previously estimate the travel curve of the rear blade to control the timing of the reset operation, but the structure of the shutter is increased in size since the photointerrupters are provided. In addition, when the photosensor is disposed near the image-pickup device, it is necessary to separately provide a device for preventing the light of the photosensor from hitting the light-receiving surface of the image-pickup device. In this manner, there is still room for improvement in the structure for appropriately setting the timing of reset operation in the apparatus formed to control the exposure time through the reset operation of the image-pickup device and the travel of the blades of the mechanical shutter.